1. Field of the Invention
The invention relates generally to radio frequency amplifiers, and more particularly, to reducing the distortion from a balanced radio frequency power amplifier.
2. Description of the Related Art
Radio frequency (RF) amplifiers are used in a wide variety of applications, including communications. Ideally the transfer function of an RF amplifier is linear, with the output of the amplifier being an amplified replica of the input to the amplifier. However, conventional RF amplifiers typically have some degree of non-linearity in their transfer function, particularly at high power levels such that different signal frequencies will be amplified by different amounts. This non-linearity in an RF amplifier produces distortion in the RF amplifier output.
Distortion in the RF amplifier output can reach levels that are unacceptable. For example, to meet the requirements of many CDMA modulation standards, power amplifier outputs need to exhibit very low distortion. If some type of linearization or distortion reduction technique is not used, the RF amplifier can have poor efficiency or require high cost components.
Many distortion reduction techniques use a distortion generator to cancel the distortion generated within a main RF power amplifier. These techniques generally assume that the low power distortion generator has a non-linear transfer function that is either similar to, or opposite from, the non-linear transfer function of the main RF amplifier. The output of the distortion generator, which exhibits the non-linear transfer function, is sampled and used to generate a signal that is combined with the output of the main RF power amplifier in an attempt to cancel the distortion present in the RF amplifier output. A drawback to this technique is that, for the signal combining to result in cancellation the non-linear transfer function of the distortion generator must match the non-linear transfer function of the main RF amplifier. This match can be difficult to achieve.
Another distortion reduction technique, the feedforward technique, overcomes the difficulty in providing a distortion generator that has a non-linear transfer function that matches the main RF amplifier. Rather, the feedforward technique samples the main RF amplifier output, which includes both the desired transmitted signal and amplifier induced distortion. The sampled output of the main RF amplifier is then combined with a sample of the amplifier input signal such that the two sampled signals destructively combine, leaving only the distortion from the main amplifier. The resulting distortion signal is then amplified using an error amplifier and is added at the output of the main RF amplifier with the gain and phase adjusted such that the error amplifier output destructively combines with the main RF amplifier output and cancels the amplifier induced distortion present in the output of the main RF amplifier. A drawback to the feedforward distortion reduction technique is that the power handling capability required of the error amplifier is based on the magnitude of the distortion. The feedforward technique can provide good performance when improving the linearity of low distortion amplifiers, however, when this technique is applied to an amplifier that is operating close to its compression point, the error amplifier power requirement may become excessively large, making this technique impractical and unsatisfactory for many applications.
From the discussion above, it should be apparent that there is a need for a system that can provide linearization of an amplifier throughout its operating range, for example when it is operating close to its compression point, without requiring excessively large error amplifier power levels.
A method and apparatus for reducing distortion in the output of a balanced RF power amplifier samples the distortion of the main RF power amplifier output and also reduces the power handling capability required for the error amplifier. A circuit that can provide these features measures the difference between the output from one portion of the balanced RF power amplifier and a sample of the amplifier input. This difference signal, which represents the amplifier distortion, can then be combined at the input to another portion of the balanced RF power amplifier. The amplitude and phase of the difference signal can be adjusted such that the distortion produced from the respective portions of the balanced RF amplifier are cancelled when the respective outputs are combined to produce the balanced RF amplifier output. Because the distortion generated in each portion of the balanced RF power amplifier should be similar, the distortion in one portion of the RF power amplifier is used to minimize the distortion in other portions of the RF power amplifier. In addition, this technique uses one portion of the balanced RF amplifier to amplify the difference signal to the desired level and thereby does not require a powerful error amplifier. Also, because the difference signal is injected into the input signal in front of the power amplifier, this technique does not require additional couplers following the main RF power amplifier output. The elimination of additional couplers after the main RF power amplifier can result in a lower cost design that provides improved efficiency.
In another aspect, the main RF power amplifier can include multiple amplifying devices that produce a combined output signal. The distortion produced in one of the amplifying devices can be sampled and used to cancel the distortion from all of the multiple amplifying devices to minimize the distortion in the main
In yet another aspect, a tone, or pilot signal, can be injected into the termination port of a balanced amplifier input splitter. The amplitude of the tone or pilot signal that is present in a balanced RF amplifier output combiner termination port can be detected and used as an input to an adaptation loop to minimize the tone, or pilot signal present in the output combiner termination port. By injecting the tone or pilot signal in the splitter termination port, the tone is isolated from the output of the balanced amplifier.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, principles of the invention.